This disclosure relates, in various embodiments, to electrostatographic imaging members. More particularly, the disclosure relates to an improved outermost exposed imaging layer, such as a charge transport layer, of an electrostatographic imaging member which extends the mechanical service life of the member.
Electrostatographic imaging members are known in the art. Typical electrostatographic imaging members include photoreceptors for electrophotographic imaging systems and electroreceptors such as ionographic imaging members for electrographic imaging systems. Generally, these imaging members comprise at least a supporting substrate and at least one imaging layer comprising a thermoplastic polymeric matrix material. In a photoreceptor, the photoconductive imaging layer may comprise only a single photoconductive layer or a plurality of layers such as a combination of a charge generating layer and one or more charge transport layer(s).
Electrostatographic imaging members can have a number of different configurations. For example, they can comprise a flexible member, such as a flexible scroll or a belt containing a flexible substrate support. The flexible member belt may be seamed or unseamed. The electrostatographic imaging members can also be a rigid member, such as those utilizing a rigid support substrate drum. Drum imaging members have a rigid cylindrical supporting substrate bearing one or more imaging layers. Although the present disclosure is equally applicable to imaging members of any configuration, for reasons of simplicity, the disclosure herein after will focus primarily on and represent flexible electrophotographic imaging members such as a flexible seamed belt.
Flexible electrophotographic imaging member belts are typically fabricated from a sheet which is cut from a web. The sheets are generally rectangular in shape. The edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheets are formed into a belt by joining overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping marginal end regions at the point of joining. Joining may be effected by any suitable means. Typical joining techniques include welding (including ultrasonic), gluing, taping, pressure heat fusing, and the like. Ultrasonic welding is generally the more desirable method of joining because it is rapid, clean (no solvents) and produces a thin and narrow seam. In addition, ultrasonic welding is more desirable because it causes generation of heat at the contiguous overlapping end marginal regions of the sheet to maximize melting of one or more layers therein to produce a strong fusion bonded seam.
A typical flexible electrophotographic imaging member belt comprises at least one photoconductive insulating layer. It is imaged by uniformly depositing an electrostatic charge on the imaging surface of the electrophotographic imaging member and then exposing the imaging member to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the imaging member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking toner particles on the imaging member surface. The resulting visible toner image can then be transferred to a suitable receiving member or substrate such as paper.
A number of current flexible electrophotographic imaging members are multilayered photoreceptors that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. In such an imaging member, the charge transport layer is the top outermost layer exposed to the environment. Since flexible electrophotographic imaging members exhibit upward curling after completion of the application of a charge transport layer, an anti-curl back coating can also be employed on the back side of the flexible substrate support (the side opposite from the electrically active layers) to achieve the desired photoreceptor belt flatness.
In normal machine design, the flexible photoreceptor belt is mounted over and around a belt support module. As such, the belt is constantly subjected to bending strain as it flexes over each of the belt module support rollers during dynamic belt cyclic motion. The greatest bending strain is tension concentrated at the surface of the charge transport layer, so that extended belt cyclic flexing has been found to facilitate the development of surface cracking. In this regard, surface cracking in the charge transport layer is somewhat unique only in belt photoreceptors and is induced, in part, due to the effect of dynamic fatigue of the belt flexing over the supporting rollers of a machine belt support module.
Surface cracking has also been found to be caused by exposure to airborne chemical contaminants as the photoreceptor segments statically “park” or directly bend over the rollers after periods of photoreceptor belt non-use during machine idling. Typical chemical contaminants include solvent vapors, environment airborne pollutants, and corona species emitted by machine charging subsystems. Surface cracking can also be exacerbated by the combination of fatigue belt flexing and airborne chemical exposure. Photoreceptor surface cracking is a critical mechanical issue seen in imaging members, particularly in flexible belts, because the cracks manifest themselves into printout defects that seriously impact copy quality.
Similarly, under normal machine electrophotographic imaging processing conditions, the top outermost exposed charge transport layer is constantly subjected to mechanical interactions against machine subsystems and components. These include, for example, sliding cleaning blade and cleaning brush actions, corona species exposure, toner debris, developer components, toner image receiving papers, and the like. Consequently, the charge transport layer is also susceptible to surface scratching, abrasion, wear, and filming which produce copy print-out defect problems as well.
Each charge transport layer-of a multi-layered photoreceptor is typically formed by a solution coating process. The coating solutions generally contain an organic solvent(s), such as methylene chloride or a chlorinated solvent. After application of the coating solution, the wet coating layer is dried at elevated temperatures to remove a substantial amount of the solvent to produce a solid layer. However, not all of the solvent may be removed from the coating layer during drying. For example, in forming a typical charge transport layer from a coating solution containing about 86 weight-% (wt-%) methylene chloride solvent and 14 wt-% dissolved solids, the solvent evaporates very quickly during the elevated temperature drying process. However, about 2 wt-% of the methylene chloride will typically still be present or trapped in the resulting charge transport layer (i.e., residual methylene chloride). The trapped solvent evaporates or “outgases” over time. The outgassing of the trapped solvent from the charge transport layer during storage and over the life of the photoreceptor causes dimensional contraction of the charge transport layer, causing increased internal strain in the charge transport layer. Thus, in addition to the bending strain induced during dynamic photoreceptor belt flexing over each belt module support roller in a machine, this increase in internal strain will exacerbate charge transport layer cracking under normal belt functioning conditions in the field.
Dimension contraction in the charge transport layer also causes the photoreceptor belt to exhibit upward curling at both edges when the belt functions in a machine. Since the contraction in belt direction is prevented by the applied tension as the belt is mounted over and around a belt support module, edge curling in the photoreceptor belt is an important issue. Edge curling changes the distance between the belt surface and the charging device, causing non-uniform surface charging density which is visible as a “smile print” defect. Such a defect is characterized by higher intensity print-images at the locations over both belt edges.
Since the charge transport layer of a typical negatively charged multilayered photoreceptor flexible belt is typically the outermost exposed layer, it is inevitably subjected to constant mechanical interactions against various electrophotographic imaging machine subsystems under a normal service environment. These mechanical interactions include abrasive contact with cleaning and/or spot blades, exposure to toner particles, carrier beads, toner image receiving substrates, etc. As a result, the charge transport layer may frequently exhibit mechanical failures such as frictional abrasion, wear, and surface cracking due to fatigue dynamic belt flexing. Under normal functioning conditions, exposure to the ozone species generated from the wires of a charging device is known to cause polymer binder chain scission, exacerbating charge transport layer cracking and wear problems. Charge transport layer wear is also an issue because wear reduces thickness and thereby alters the equilibrium of the balancing forces between the charge transport layer and the anti-curl back coating, impacting imaging member flatness. Moreover, in a rigid electrophotographic imaging member drum design utilizing a contact AC Bias Charging Roller (BCR), ozone species attack on the charge transport layer polymer binder is more pronounced because of the close vicinity of the BCR to the charge transport layer of the imaging member drum. As a consequence, charge transport layer wear is a serious problem which significantly reduces the functional life of the imaging member.
To resolve one or more of the above-noted shortcomings and issues, a method of fabricating electrophotographic imaging members to produce robust mechanical charge transport layer function has been investigated and successfully demonstrated as described below. The imaging members produced thereby exhibit good wear resistance, cracking life extension, and durability. Such imaging members exhibit enhanced physical/mechanical service life.